BUG/MEDIUM: listener: make sure we don't pick stopped threads

BUG/MEDIUM: listener: make sure we don't pick stopped threads

Dragan Dosen reported that after the multi-queue changes, appending
"process 1/even" on a bind line can make the process immediately crash
when delivering a first connection. This is due to the fact that I
believed that thread_mask(mask) applied the all_threads_mask value,
but it doesn't. And in case of even/odd the bits cover more than the
available threads, resulting in too high a thread number being selected
and a non-existing task to be woken up.

No backport is needed.